In reciprocating engines (reciprocating piston engines), except for some 2-cycle engines, there are air-intake valves and exhaust valves that open and close in synchronization with the rotation of the crankshaft. Also, there is a cam follower inside the valve mechanism of the engine that converts the rotation of the camshaft to the reciprocating motion of the valve stem (air-intake valve and exhaust valve). In this kind of reciprocating engine, the motion of the camshaft that rotates in synchronization with the rotation of the crankshaft (the rotating speed of the camshaft is ½ that of the crankshaft in the case of a 4-cycle engine) is transmitted to the air-intake valve and exhaust valve by the rocker arm of the cam follower to move the air-intake valve and exhaust valve in a reciprocating motion in the axial direction.
In order to secure the strength of the rocker arm inside the valve mechanism of the engine, while at the same time make it more lightweight, it has been proposed and put in practice to manufacture the rocker arm by press-working metal plate such as steel plate. Of this kind of cam follower having a sheet-metal rocker arm, FIGS. 8 thru 11 show a cam follower that is disclosed in U.S. Pat. No. 5,048,475. This cam follower comprises a sheet-metal rocker arm 1, roller 2 and pivot 3, where the roller 2 is supported by the pivot 3 such that it rotates freely with respect to the sheet-metal rocker arm 1.
The sheet-metal rocker arm 1 is associated with the valve stem 7 of the air-intake or exhaust valve (not shown in the figure), the plunger 8 of the rush adjuster, which is the center of the rocking motion of the sheet-metal rocker arm 1, and the camshaft 9. The sheet-metal rocker arm 1 is made from a metal plate such as a 2 mm to 4 mm thick steel plate by a punching process to remove any unnecessary parts, and plastic-working, such as drawing, for obtaining the desired shape; and it comprises a pair of side-wall sections 4 and first and second connecting sections 5, 6 that connect both of these wall sections 4 together, respectively. Of the connecting sections 5, 6, the first connecting section 5 comes in contact against the base end face of the valve stem 7 and functions as a pressure portion for displacing the valve stem 7, and the second connecting section 6 functions as a fulcrum portion for coming in contact with the tip end face of the plunger 8. Therefore, in the example shown in the figures, a spherical concave section is formed on one end surface (lower surface in FIG. 10) of the second connecting section 6. Construction that differs from that of the example shown in the figure, in which a screw hole is formed in the section that corresponds to the second connection section, so as to threadably receive an adjust screw with a spherical surface end for fixing, is also known.
On the other hand, the roller 2 is located between the pair of connecting sections 5, 6, and supported by the pivot 3 by such that it can rotate freely. In order to support the roller 2, both end sections of the pivot 3 fit in the through-holes that are formed at matching locations in the pair of wall sections 4. The outer peripheral edge sections of both end surface of this pivot 3 are crimped outward toward the peripheral edge sections of each of these through-holes. With this construction, both end sections of the pivot 3 are attached to the pair of wall sections 4 such that the pivot 3 spans between both of these wall sections 4. The roller 2 fits around the middle section of the pivot 3 that spans between both of these wall sections 4 in this way, and is supported either directly or by way of a radial needle roller bearing such that it can rotate freely.
As shown in FIG. 11, when installed in the engine, one surface of the first connecting section 5 (bottom surface in FIG. 11) comes in contact with the base end face of the valve stem 7, and the tip end face of the plunger 8 comes in contact with the spherical concave section on one surface of the second connecting section 6, and the outer peripheral surface of the cam 9 securely fastened in the middle section of the cam shaft comes in contact with the outer peripheral surface of the roller 2. When the engine is running, as the cam 9 rotates, the sheet-metal rocker arm 1 moves in a rocking motion with the point of contact between the tip end surface of the plunger 8 and the spherical concave section as the center (fulcrum), and the pressure force from the first connecting section 5 and the elastic force of a return spring 10 moves the valve stem 7 in a reciprocating motion in the axial direction. Incidentally, a cam follower with a sheet-metal rocker arm having similar construction is also disclosed in Japanese Patent Publication No. Tokukou Hei 6-81892, which is not shown in the figures here.
The thickness of the sheet-metal rocker arm 1 made by plastic-working of sheet-metal changes during the plastic-working process, so if the shape and construction of the other parts are not designed properly, it may not be possible- to secure sufficient durability. This aspect is explained using FIGS. 12 thru 14 in addition to FIGS. 8 thru 11, mentioned above.
When a sheet-metal rocker arm 1 like that shown in FIGS. 8 to 11 is manufactured by drawing of a metal plate such as a steel plate, in a normal processing method, with regard to both end sections in the width direction (top and bottom direction in FIGS. 10 and 11) of the pair of side-wall sections 4, the end sections on the side of the first and second connecting sections 5, 6 (top side in FIGS. 10 and 11) stretch in the planar direction an amount more than the end sections on the other side (bottom side in FIGS. 10 and 11) and thus the thickness becomes thinner in the end sections on the side of the first and second connecting sections 5, 6. That is, as shown exaggeratedly in FIGS. 12 and 14, the cross-sectional shape in the width direction of both of the side-wall sections 4 is a wedge shape that is inclined in a direction such that it becomes thicker moving away from the connecting sections 5, 6 (going lower in FIGS. 12, 14). On the other hand, the inner side surfaces of these sidewall sections 4 must be parallel with each other. The reason for that is to prevent that only one of these side wall sections 4 comes into contact with the roller 2 located between these sidewall sections 4, so that the roller 2 can rotate smoothly between the side wall sections 4.
When the inner side surfaces of these side-wall section 4 having a wedge-shaped cross-sectional shape are arranged such that they are parallel with each other, the outer side surface of the side-wall sections 4 are not parallel with each other as shown in FIGS. 12 and 14. That is, the space between the outer side surfaces of these sidewall sections 4 gradually becomes large as it goes away from the connecting sections 5, 6 (to the bottom in FIGS. 12 an 14). The space between the outer side surfaces of the sidewall sections 4 in this way similarly gradually changes in the middle section in the width direction of these sidewall sections 4 where through holes 11 are formed for attaching both ends of the pivot 3. For example, in the results of the tests and measurement performed by the inventors, the thickness of the side-wall sections 4 was approximately 1 mm along the edge on the side of the connecting sections 5, 6 (top edge in FIGS. 12 and 14), and was approximately 3 mm along the edge on the opposite side (bottom edge in FIG. 12). In this case, the thickness of the edge of the through holes 11 was 2.3 mm on the side of the connecting sections 5, 6 and was 2.9 mm on the opposite side. The difference in this thickness is the degree that the outer side surfaces of the sidewall sections 4 are not parallel.
In this state with the outer side surfaces of the sidewall sections 4 are not parallel with each other, it is not possible to uniformly crimp and fasten both end sections of the pivot 3 all the way around the beveled sections 12 formed around the peripheral edges of the openings of each through hole 11. In other words, since both end surfaces of the pivot 3 are at right angles with the center axis of the pivot 3, the positional relationship in the axial direction between both of these end surfaces and the beveled sections 12 is not uniform in the circumferential direction. In order to maintain sufficient crimping strength, it is necessary to have a proper positional relationship in the axial direction between both of the end surfaces and the beveled sections 12. However, as long as the outer side surfaces of the side-wall sections 4 are not parallel with each other, it is not possible to have a proper positional relationship all the way around the openings. Incidentally, it is unrealistic from the aspect of mass production to make both end surfaces in the axial direction of the pivot such that they are not parallel with each other in alignment with the outer side surfaces.
Therefore, conventionally, the positional relationship in the axial direction between the beveled sections 12 formed around the peripheral edges of the through holes 11 and both end surfaces of the pivot 3 is made to be proper on the opposite side from the connecting sections 5, 6 (lower side in FIG. 12), as shown in FIG. 12. Also, as shown by the dot-dash line α in FIG. 13, a crimping tool (punch) is pressed on a portion of the end surfaces of the pivot 3 from the middle to the side opposite to the connecting sections 5, 6, so that the edge of the portion from the middle to the side opposite to the connecting sections 5, 6 is crimped outward in the radial direction. Therefore, as shown in FIG. 14, the outer peripheral surface around the end section of the pivot 3 comes in contact with the inner peripheral surfaces of the through holes 11 at a section on the side closer to the connecting sections 5, 6 (on the upper side in FIG. 14). On the sides where the crimped sections 13 are formed, or in other words, on the sides opposite from the connecting sections 5, 6 (on the lower side in FIG. 14), there is a clearance 14 between the outer peripheral surface around the end sections of the pivot 3 and the inner peripheral surface of the through holes 11.
On the other hand, disclosed in Japanese Patent Publication No. Tokukai Hei 3-172506 is a technique for improving the manufacturing process of the sheet-metal rocker arm so as to keep the difference of the thickness in the width direction of the pair of sidewall sections, that support both ends of the pivot, to a minimum. In the case of this prior technique, first, a first intermediate blank 15 is made as shown in FIG. 15(A) by plastically deforming a piece of metal plate that will become the blank. Then, by performing a punching process on part of this first intermediate blank 15, a second intermediate blank 17 is formed having a hand-drum shaped through hole 16 in it as shown in FIG. 15(B). Next, a bending process is performed on both side sections of the through hole 16 of this second intermediate blank 17, so that the both side sections are raised to form a third intermediate blank 18 having a pair of side-wall sections 4a that are parallel with each other.
Furthermore, as shown in FIG. 16(A) and FIG. 16(B), through holes 11 are formed in alignment with each other on the side-wall sections 4 of this third intermediate blank 18, and a roller 2 is provided around the outer peripheral surface in the middle of a pivot 3, whose ends are both supported in the through holes 11. The roller 2 is supported by a radial needle roller bearing 19 such that it can rotate freely, to form the cam follower having a sheet-plate rocker arm. In the case of the invention disclosed in Japanese patent Publication No. Tokukai Hei 3-172506, the outer peripheral edges of both end surfaces in the axial direction of the pivot 3 are crimped outward all the way around. Therefore both end sections in the axial direction of the pivot 3 are supported inside the through holes 11 such that they are nearly concentric with the through holes 11.
In the first example of prior art construction shown in FIG. 14, since there is a clearance 14 between the outer peripheral surface around the end sections of the pivot 3 and the inner peripheral surface of the through holes 11 on the opposite side from the first and second connecting section 5, 6, the crimped sections 13 formed on the ends of the pivot 3 support the load applied to the pivot 3 from the cam 9 shown in FIG. 11 by way of the roller 2 (further by way of the radial needle roller bearing). In other words, when the engine is running, a load is applied to pivot 3 from the top side toward the bottom side in FIG. 14 (in balance with the elastic force of the return spring 10). Since there is a clearance 14 between the outer peripheral surface around the end sections of the pivot 3 and the inner peripheral surface of the through holes 11 in the direction where the load is applied, the crimped section 13 supports the load, and the load is not directly transmitted from the outer peripheral surface around the end sections of the pivot 3 to the inner peripheral surface of the through holes 11.
However, the area of contact between the crimped sections 13 and the beveled sections 12 is small, and since the crimped sections 13 are formed just by plastically deforming the ends of the pivot 3, it is easy for them to become plastically deformed. Therefore, after a long period of use, the crimped sections 13 plastically deform inward in the radial direction, and there is a possibility that the contact pressure between the crimped sections 13 and the beveled sections 12 will decrease. When the contact pressure decreases in this way, the pivot 3 and the roller 2 that is supported around the middle section of the pivot 3 are lashed with respect to the sheet-metal rocker arm 1, and thus vibration and noise occur so largely while the engine is running, which is not desirable.
In the case of the second example of prior art construction shown in FIG. 16, both ends in the axial direction of the pivot 3 are supported nearly concentrically radically inside the through holes 11, so that on the side where radial loads are supported, the thickness of the clearances between the outer peripheral surfaces around the ends in the axial direction of the pivot 3 and the inner peripheral surfaces inside the through holes 11 can be made smaller than in the case of the first example shown in FIG. 14. However, there is no secure direct contact between the outer peripheral surfaces around the ends in the axial direction of the pivot 3 and the inner peripheral surfaces inside the through holes 11 on the side where the radial loads are supported, so there is still a possibility that lost motion will occur due to plastic deformation of the crimped sections.
In JP patent publication No. Jitsuko Hei 4-44289, construction is disclosed in which the crimping position is regulated, so that the outer peripheral surfaces around the ends of the pivot come in contact with the inner peripheral surfaces of the through holes at the sections where the radial load is supported. However, the construction described in this disclosure is for a rocker arm made by casting, which differs from the cam follower of this invention having a lightweight and low cost sheet-metal rocker arm.
The cam follower of this invention was invented taking the aforementioned problems into consideration.